1. Field of the Invention
This invention relates to a method for displaying gradation with a plasma display panel (hereinafter referred to as"PDP").
2. Description of the Related Art
One such method is disclosed, for example, in the paper of the image engineering study group of The Institute of Electronics, Information and Communication Engineers, IT 72-45 (1973). In this paper, the gradation display is performed by time-dividing a field of an image into a plurality of subfields, and giving a proper weight on a luminescent period in each subfield. Thus, a linear gradation characteristic is obtained by altering a luminescent period to display a halftone in a PDP, which utilizes a discharge luminescence and in which a current or a voltage is not proportional to a luminescence.
FIG. 7 shows an example of a conventional method for displaying gradation with a PDP disclosed in JP-A-4-195188. In this method, a subfield is further divided into an addressing period and a sustaining period. In the addressing period, a binary data, i.e., on or off is written into every pixel by noninterlace scanning in which all scanning electrodes are selected sequentially. In the sustaining period following the addressing period, all pixels that have been given the on data are held emitting light for a predetermined period to display an image in a binary gradation.
Furthermore, the weight given to the sustaining period of each subfield, i.e., a ratio of the sustaining periods may be set 1, 2, 4, 8 . . . , 2.sup.n-1 (n is a number of subfields) and all images in the subfields included in a field may be accumulated in eyes of a viewer. Thus, an image can be displayed in 64 gradation steps when n =6, or in 256 gradation steps when n =8.
FIG. 8 shows another example of a conventional method for displaying gradation disclosed in Japan Television Institute Memoir Vol. 38, No. 9 (1984). In this method, one field is divided into a plurality of subfields in the same way as the above-mentioned method shown in FIG. 7. However, the method shown in FIG. 8 starts the sustaining period immediately after selecting one of the scanning electrodes to write data into it. This operation is different from the method shown in FIG. 7. The next scanning electrode to be selected is given data by utilizing a stop period for the light emitting pulse. The sustaining period of each subfield is given weight 2.sup.m-1 (m =1, 2 . . . , n) for example in the same way as the example shown in FIG. 7.
By such a method for displaying gradation, a PDP can display an image with a sufficient number of gradation levels, and it has attracted attention as realizing a so-called wall-hung TV or a flat TV in recent years.
However, the above mentioned method has the following disadvantage. The majority of the time is used for the addressing period for writing data and the sustaining period is too short to obtain a sufficient brightness of the PDP. The current mainstream is a surface discharge AC type PDP, which needs a period of approximately 2.5 microseconds for selecting a scanning electrode and writing data. In this case, if a PDP having 500 scanning electrodes is driven with 8-subfield division, the addressing period is 10 milliseconds (2.5 microseconds .times.500 .times.8). Therefore, only 6.7 milliseconds remain for the sustaining period in one field (16.7 milliseconds). As a result, the brightness of a PDP may be insufficient in the method of the prior art.